The present invention relates to multi-electron beam image display devices and more particularly to means for controlling brightness uniformity in such devices by pulse width modulation of the multi-electron beam source.
Recently, many display devices have been suggested utilizing a plurality of electron beams, each of which illuminates a separate region of the entire display. One of the difficulties encountered in the use of separate electron beams for different portions of the display is that each beam will vary in terms of its electron current. This variation causes nonuniformity in the brightness of the displayed image. In order for such devices to be practical, the display must have a uniform brightness.
One method for achieving uniform brightness is described in U.S. Pat. No. 4,121,137 issued Oct. 17, 1978 to T. L. Credelle entitled "System for Achieving Image Uniformity in Display Devices." The Credelle system utilized a collector to sense the electron current of each beam. The level of the electron current of each beam is stored in a memory. When a particular beam is to be modulated, the memory is addressed so that the electron current level information for that beam is read out. The stored information and the incoming image element brightness information are combined to modulate the particular electron beam. One drawback of the Credelle system is that the collector is located in such a position within the display device that it can only sample the beam current before image information is displayed and during the vertical retrace time. The Credelle system does not permit continuous correction for brightness non-uniformities during operation. Another drawback of the Credelle system is that by sensing the current level in each beam, an inaccuracy is introduced into the correction system when the beam current level is very low.
Another method for achieving uniform brightness by controlling each electron gun is described in U.S. Pat. No. 4,126,814 issued Nov. 21, 1978 to F. J. Marlowe entitled "Electron Gun Control System." The Marlowe system uses a collector to sense the electron current from each gun. A comparator compares the sensed beam current signal with a brightness reference signal. The output of the comparator controls the generating of digital words. These digital words, which are stored in a memory, comprise mapping information to convert various brightness levels to voltages. During the display of an image, the video signal containing brightness information combines with the mapping information from the memory to bias the electron gun grids in order to generate electron beams which will illuminate the screen to the desired brightness level. Although the electron gun characteristics may vary from gun to gun of the display, the use of the reference brightness signal and the memory ensure that equalized beam currents will be generated for various brightness levels. The Marlowe system shares a common drawback with the Credelle system in that the collector is located in such a position within the display device that it can only sample the beam current before image information is displayed and during the vertical retrace of the display. Thus, neither the Marlowe system nor the Credelle system permit continuous correction for brightness non-uniformities during the display of image information.
A third method for achieving uniform brightness is described in U.S. Pat. No. 4,077,054 issued Feb. 28, 1978 to J. G. Endriz entitled "System for Modulating a Flat Panel Image Display Device." The Endriz system continuously senses an anode voltage proportioned to the instantaneously charge from electron beams impinging on the cathodoluminescent screen and compares the sensed voltage to a predetermined voltage waveform generated from the incoming video signal. A bias voltage corresponding to the relationship between the sensed voltage and the waveform voltage biases a modulation electrode to either increase or decrease electron flow to the anode. The resulting change in the magnitude of the negative charge build up on the anode produces a change in the bias voltage. For example, when the sensed anode voltage is less negative than the waveform voltage, the comparator biases the modulation electrodes so as to increase the electron flow thereby increasing the buildup of negative charge on the anode and decreasing the anode voltage. The Endriz system provides a feedback mechanism for controlling the flow of electrons so that a predetermined level of brightness set by the amplitude of the waveform voltage is obtained. A drawback of the Endriz system is that when the beam current level is low, inaccuracies are introduced into the correction system.